Production of biofuel from tobacco plants

ABSTRACT

A method of producing biofuel from tobacco biomass including solvent extraction of the tobacco biomass with methyl acetate or ethyl acetate, transesterification of the oil obtained from the biomass and separation of the biofuel from the transesterified product. Excellent yields of biofuel based on the weight of the biomass are obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for extracting lipids or oils from plant biomass. In particular, the invention is directed to methods of obtaining biofuel from tobacco plants.

2. Description of the Related Technology

Humans have a long history of cultivating tobacco, first as an ornamental plant and a medical plant, later as a luxury product. For most of recent history, human consumption of tobacco has been based on the transformation of the leaves into smoking products, inhaling powders and chewable products. Recently, alternative uses of tobacco have been proposed, such as for the production of alimentary proteins through purification from leaves, for the provision of pharmacologically useful active ingredients normally present in the leaves, and as a source of recombinant proteins expressed in the leaves or seeds of genetically modified plants. Another promising use for tobacco is as a source of fuel for the production of green energy.

The negative consequences, in environmental terms, of the use of fossil fuels and their limited availability have provided an incentive for the search for new energy sources. Amongst these, biofuels are a possible choice due to their renewability. Biofuels of agricultural origin are mostly bioethanol made from simple (i.e. saccharose) or complex (i.e. cellulose) sugar producing plants. Model plants for such production have been identified in sugar cane, corn, wheat, potato, tapioca, sugar beet, barley, sorghum etc. Alternatively, technology has been developed to produce fuel oil and biodiesel from oleaginous or non-oleaginous species but rich in oil such as soybean, sunflower, rape, peanut, flax, corn, sesame, palm, palm-kernel, coconut, ricinus etc.

Biodiesel as alternative energy source has some advantages over bioethanol. One advantage is that the dominant starting material for obtaining bioethanol in the U.S. is corn and soybeans, which are staple food crops. In addition, bioethanol production is typically less efficient than biodiesel production. For example, corn ethanol yields only 25% more energy than the energy invested in its production, whereas biodiesel may yield up to 93% more energy than need be invested in its production. Lastly, bioethanol produces more greenhouse gases than biodiesel. Relative to fossil fuels, greenhouse gas emissions are reduced up to 12% by the production and combustion of bioethanol and up to 41% by production and use of biodiesel. Biodiesel also releases fewer air pollutants per net energy gain than bioethanol (Hill et al., Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels, Proc Natl Acad Sci USA. 2006, 103(30):11206-11210).

Biodiesel from tobacco could be an important supplement to our nation's renewable energy strategy. The current trend focuses on extracting oil from tobacco seeds. With production of tobacco worldwide estimated at over four million hectares, tobacco seeds may be a viable alternative source for energy. However, tobacco biomass, especially the leaves, has been essentially overlooked for biofuel production. Tobacco plants can yield around 170 tons/acre of low-cost, high-value biomass materials without a high labor requirement, chemical inputs, or geographic restrictions. The tobacco plant has a very large leaf area, a small inflorescence and a ratio of aerial part:roots that is the highest observed among agricultural plants. Similar to hardwood trees, tobacco will coppice or re-sprout from its stump after it has been cut. Coppicing makes multiple harvests in a year possible, enabling it to produce very high biomass tonnage. In addition, tobacco thrives on different kinds of soil in a wide range of environments. Finally, since tobacco is a non-food plant that can thrive in poor soil, it does not compete with food-producing plants such as corn and soybeans for more fertile soil.

U.S. 2010/0184130 discloses a genetically engineered tobacco plant and a method of extracting oil from its biomass. Genetic engineering can increase oil deposits in tobacco leaves. This process uses a modified hexane extraction method to retrieve oil from biomass.

U.S. 2009/0234146 discloses methods of extraction and transesterification of oil from biomass, such as plants or algae. The extraction methods involve treating biomass with a co-solvent system, which comprises at least one polar covalent molecule and at least one ionic liquid. The polar covalent molecule may be methyl acetate. The transesterification of the extracted oil is done using methanol with sodium hydroxide as a catalyst, or methanol with sulfuric acid as a catalyst. However, the reference does not mention tobacco.

The present invention is aimed at increasing the efficiency of extracting oils from tobacco biomass and treatment of the oil so that it will be suitable for use as biodiesel.

SUMMARY OF THE INVENTION

The present invention is directed to a method of producing biofuel from tobacco biomass including the steps of extracting the tobacco biomass with a polar organic solvent, separating the extracted oil from the polar organic solvent, transesterifying the extracted oil and separating the biofuel from the reaction mixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

For illustrative purposes, the principles of the present invention are described by referencing various exemplary embodiments. Although certain embodiments of the invention are specifically described herein, one of ordinary skill in the art will readily recognize that the same principles are equally applicable to, and can be employed in other systems and methods. Before explaining the disclosed embodiments of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of any particular embodiment shown. Additionally, the terminology used herein is for the purpose of description and not of limitation. Furthermore, although certain methods are described with reference to steps that are presented herein in a certain order, in many instances, these steps may be performed in any order as may be appreciated by one skilled in the art; the novel method is therefore not limited to the particular arrangement of steps disclosed herein.

It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Furthermore, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. The terms “comprising”, “including”, “having” and “constructed from” can also be used interchangeably.

In a first aspect, the present invention is related to the production of biofuel from biomass, particularly from tobacco. In this aspect, oil in tobacco biomass is first extracted from the biomass using a polar organic solvent. The oil is then transesterified to produce fatty acids that are suitable for use as biofuels.

As used herein, the term “biomass” refers virtually any tobacco plant-derived organic matter. This includes whole plants, plant organs (i.e., leaves, stems, flowers, roots, etc.), seeds, plant cells (including tissue culture cells), progeny of one or more of the foregoing and comminuted forms of such materials.

Any species or type of tobacco plant may be use for the present invention. Tobaccos having higher oil or lipid contents in their biomass are a preferred type of tobacco for use in the present invention. For example, Navajo Mountain tobacco, which has a relatively high oil content in its biomass, is a preferred type of tobacco for use in the present invention. Tobacco plants genetically engineered to have high oil or lipid contents in their biomass may also be used. Suitable genetically engineered tobacco plants include those described in, for example, Andrianov et al., “Tobacco as a production platform for biofuel: overexpression of Arabidopsis DGAT and LEC2 genes increases accumulation and shifts the composition of lipids in green biomass,” Plant Biotechnol J., Vol. 8, pages 277-87 (2009); and U.S. 2010/0184130).

In some embodiments of the invention it may be desirable to grow the tobacco plants hydroponically in order to speed growth, produce higher yields and/or grow the plants year round. Hydroponic cultivation allows growth of the tobacco plants under highly controlled, reproducible conditions, and facilitates efficient harvest of the extensive, filamentous root system in a clean, intact condition.

One exemplary process for hydroponic growth of tobacco is as follows. Tobacco seeds are allowed to germinate at or near the surface of a moist plant potting mixture. Suitable conditions are a temperature of about 80° F. and a 60% relative humidity. About two weeks after seed germination, seedlings are thinned (removed) to leave sufficient room for unhindered growth of the remaining seedlings to a stage at which they are about six inches tall, and have about six leaves. When the seedlings reach a height of about six inches they are typically transplanted, with the root system and pellet of the potting material intact, into a hydroponic device containing a suitable nutrient solution and a means for aeration (oxygenation) of the nutrient solution. The hydroponic device should also provide for replenishment of the dissolved nutrients, and should be of a size sufficient to accommodate a fully-grown tobacco plant.

Other suitable growth methods may be employed. For example, the tobacco may be grown in soil provided with additional growth media, such as coconut fiber. The tobacco may preferably be grown in a greenhouse though for large scale production, growth outdoors in a suitable climate will typically be employed.

When full-grown, the tobacco plants are harvested and the oil and/or lipids are extracted from the biomass. Various parts of the tobacco plants, including the stems, roots, leaves and/or seeds may be uses for the extraction step of the present invention. One skilled in the art will also appreciate that the conditions of the extraction procedure may be adjusted as needed in order to optimize it for the type or types of biomass used.

The tobacco biomass may be pretreated prior to the extraction step. The pre-treatment steps can include, but are not limited to, one or more of separation of the biomass from growth media, drying of the biomass and physical and mechanical comminution of the biomass to increase its surface area. Any method known to one of skill in the art can be used to carry out the pre-treatment steps. For example, the biomass can be separated from growth media by centrifugation, rinsed with deionized water to further remove traces of growth media, dried under vacuum or freeze dried.

The extraction step may be carried out by contacting the tobacco biomass with a polar organic solvent to form an extraction mixture to thereby extract the oil and/or lipid component of the biomass into the solvent. Once the desired amount of extraction is complete, the remaining biomass is separated from the extraction mixture.

The polar organic solvent may be selected from methyl acetate and ethyl acetate.

Any suitable amount of organic solvent may be employed for the extraction process. For example, amounts of 1-4 grams of organic solvent per gram of biomass may be employed. The manner in which the biomass and the polar organic solvent are combined to form the extraction mixture is not critical and thus the biomass may be added to the polar organic solvent or the polar organic solvent may be added to the biomass.

After the extraction mixture is formed, it may be held at ambient temperature, e.g. 15-25° C. for a sufficient time to extract a substantial portion of the oil or lipids from the tobacco biomass. The extraction mixture may optionally be heated with stirring to temperatures up to the boiling point of the lowest boiling point polar organic solvent in the extraction mixture. The duration of the extraction can be between about 1 hour to about 48 hours, depending on the type and amount of biomass, type of solvent and whether steps are taken such as heating or other means to facilitate the extraction. A skilled person may determine the optimal duration of extraction by simple experiments. Preferably, the extraction is carried out at a temperature of from about 60 to about 65° C. for a period of about 16 to about 48 hours.

In some embodiments, the extraction of the tobacco biomass with the polar organic solvent can be enhanced through the application of sonication, agitation, pressure, and/or radiation energy (e.g., microwave, infrared) to increase the rate and efficiency of extraction.

After extraction is substantially complete, as determined on a case-by-case basis, primarily based on the economics of the extraction process, the extraction mixture may be subjected to additional steps to facilitate separation of the extracted oils from the extraction mixture. For example, centrifugation or filtration may be used to remove the remaining biomass from the extraction mixture. The polar organic solvent can also be removed from the extraction mixture by, for example, rotary evaporation or distillation such as vacuum distillation.

Oil extracted from the tobacco biomass is not optimal for direct use in combustion engines. Thus, the extracted oil is transesterified by reaction of the oil with a low molecular weight alcohol, such as methanol or ethanol, in the presence of a catalyst. The transesterification reaction involves exchange of ester groups and thus produces two products: fatty acid esters (the component useful as biodiesel) and glycerine (a valuable byproduct usually sold to be used in soaps and other products). Another by-product of the reaction is a natural pesticide source that can be separated and sold separately as another product of the process. Caustic compounds and water may be added to the tobacco oil before carrying out the transesterification step in order to carry out a conventional “alkali refining” step.

The transesterification process may be carried out by mixing the oil with a low molecular weight alcohol, such as methanol, in the presence of a catalyst to form a reaction mixture. The transesterification may be either an acid-catalyzed transesterification or a base-catalyzed transesterification step. The catalyst used in the transesterification step may be an acidic catalyst or a basic catalyst. The acid catalyst can be a Bronsted acid that is a sulfonic or sulfuric type acid, H₂SO₄, HCl, acetyl chloride, BF₃, and the like. The base catalyst can be, for example, KOH, NaOH, NaOCH₃, Na₂CH₂CH₃, guanidines (such as, for example, 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD)); and metal complexes of the type M(3-hydroxy-2-methyl-4-pyrone)2(H₂))₂ where M=Sn, Zn, Pb, or Hg. K₂CO₃ may also be used, which produces a methanol soluble KOCH₃, as well as ethanol insoluble KHCO₃. In one exemplary embodiment, K₂CO₃ is employed in an amount of 6% of oil mass for the transesterification process.

The transesterification reaction is typically carried out for at least 30 minutes at a temperature below the boiling point of the alcohol (usually at about 65° C.). As the alcohol and oil components have only limited miscibility in each other, the biphasic reaction mixture is intensely stirred and/or a phase transfer catalyst may be used in order to accelerate the transesterification reaction. Generally, acid catalysts require longer transesterification reaction times than base catalysts. In preferred embodiments, acid catalysts are used in combination with a larger amount of alcohol in the transesterification reaction mixture than when base catalysts are used.

The alcohol suitable for use in the transesterification process may be any low molecular weight alcohol preferably having no more than four carbon atoms, such methanol or ethanol. The alcohol is typically employed in the reaction mixture in large excess relative to the amount of oil in order to drive the reaction in favor of fatty acid ester production. The molar ratio of alcohol to transesterifiable oil in the reaction mixture should be at least about 3:1, and up to 6:1. The ratio may vary depending on the base that is used.

When the transesterification reaction mixture reaches a point that is close to equilibrium, generally at a conversion rate of about 80% of the glyceride esters in the reaction mixture relative to the total fatty acid esters, the reaction mixture may be allowed to settle for about 12-24 hours. Thereafter, the non-polar phase may be separated and the reaction may be optionally repeated using freshly admixed alcohol and catalyst. After this optional second transesterification step is carried out, the upper non-polar phase (biodiesel phase) is separated, and optionally subjected to 9 evaporation or distillation to remove alcohol therefrom.

If desired, conventional fuel additives, such as additives for improving cold resistance, combustion, storage stability, etc., may be added to the resulting biofuel.

The transesterification can be enhanced through the application of sonication, gentle heating, agitation, pressure, and/or radiation energy (e.g., microwave, infrared). For example, the multi-phase reaction mixture may be heated with stirring to a temperature below the boiling point of the alcohol.

The duration of the direct transesterification reaction can be between from about 1 hour to about 48 hours or more, until the direct transesterification reaction is substantially complete. A substantially complete reaction is preferably one in which no significant further increase in the amount or concentration of product can be obtained by further reaction under the reaction conditions employed. After substantial completion of the direct transesterification reaction, the multi-phase reaction mixture can be subjected to additional processes to facilitate further separation of the reaction products from each other. For example, the multi-phase reaction mixture can be centrifuged, and the component comprising the fatty acid ester product can be removed by, for example, decanting or pipetting into a separate container.

In some embodiments, the fatty acid ester-containing product can be purified by extraction with a solvent such as a non-polar solvent, such as hexane or a mixture of isopropanol and hexane at a molar ratio of 5:4, after transesterification. Some base, such as sodium hydroxide, may be added to facilitate this separation. In the event that such a solvent extraction is employed, the product may subsequently be separated from the solvent by, for example, evaporating the solvent from the product under vacuum.

The fatty acid ester-containing product can optionally be washed with water. In one embodiment, this product is mixed with an equal portion of distilled water and allowed to stand for 24 hours, and separated from the water. Anhydrous sodium sulfate or another suitable water absorbing material can be used to absorb any remaining water from the fatty acid ester-containing product.

The polar organic solvent may optionally be recovered and recycled for use in another direct transesterification reaction or extraction process. Solvent recovery can be conducted by, for example, centrifugation of the reaction mixture to pellet the treated biomass and decanting the solvent and rotary evaporation. The polar organic solvent can also be recovered by mechanical filtration in which a series of mesh filters with incrementally decreasing pore size are employed.

The catalyst used in the transesterification step may be an acidic catalyst or a basic catalyst. The acid catalyst can be a Bronsted acid. The acid catalyst may be selected from a sulfonic or sulfuric acid such as H₂SO₄, HCl, acetyl chloride, BF₃, and the like. The base catalyst can be, for example, KOH, NaOH, NaOCH₃, Na₂CH₂CH₃, guanidines (such as, for example, 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD)); and metal complexes of the type M(3-hydroxy-2-methyl-4-pyrone)2(H₂))₂ where M=Sn, Zn, Pb, or Hg.

In an exemplary example, acetyl chloride can be used as a catalyst in a transesterification reaction with methanol. The catalytic effect is a two-step reaction in which the acetyl chloride first reacts with the methanol to form methyl acetate and gaseous hydrogen chloride that dissolves in the methanol. The hydrogen chloride then protonates the carbonyl oxygen of the glyceride, facilitating the exchange of the ester groups in the glyceride.

A combination of one or more of the selection of a particular type of tobacco, a particular set of growth conditions, e.g. hydroponic growth, a particular selection of extraction solvent and use of a transesterification reaction produces a relatively high yield of biofuel relative to the total plant mass. In particular, use of Navaho Mountain tobacco grown under hydroponic conditions, extracted with methyl or ethyl acetate and subjected to transesterification provided a particularly high yield of biofuel relative to total plant mass.

The biofuel product typically contains a high percentage of fatty acid esters, which are the desired fuel products. The bonds present in the fatty esters may be determined by infrared analysis. For example the presence of methanol and double bonds (either trans or cis) in the hydrocarbon chain of the biofuel may be determined in this manner. Also, the SP2 carbon atoms may be identified in this manner. Infrared analysis may also be used to determine the composition of methyl esters in the product.

Thin layer chromatography (TLC) may be used to determine the amount of unreacted triglycerides in the composition. The solvent used for TLX may be, for example, a mixture of 85 ml of hexane, 15 ml ethyl ether and 1 ml HC₂H₃O₂.

Viscometry may be used to measure the relative viscosities. This may be done by comparing the viscosity of the product to control oil.

The combustion performance of the biofuel may be determined, for example, by using a Bomb Calorimeter.

The following non-limiting examples are provided to further illustrate embodiments of the present invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples that follow represent approaches discovered by the inventors to function well in the practice of the invention, and thus can be considered to constitute examples of modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments that are disclosed and still obtain a like or similar result without departing from the scope of the invention.

EXAMPLE 1 Extraction

Tobacco leaves (9.2000 g) were dried before oil extraction. Leaves were ground using a 19,000 rpm commercial grinder. The resultant pulp was placed in a porcelain thimble with methyl acetate. The solvent became darker with time until a medium dark green color was observed after about three hours. Extraction followed using a Soxhlet extractor for two hours. The tobacco oil was recovered with solvent in a round-bottomed flask. Distillation followed to remove the solvent. Sodium sulfate was used to remove any moisture in the tobacco oil. 5.00 ml oil was obtained. As a comparative example, a rotary evaporator was used for solvent removal and the extraction solvent was methyl acetate and hexane.

EXAMPLE 2 Transesterification

3.028 g of tobacco oil was mixed with 10 ml of 1% H₂SO₄ as catalyst, 10 ml of methanol, and 40 ml of a 5:4 mixture of isopropanol (70%)/hexane. In a variation, CH₃OH/oil is mixed with 5% H₂SO₄ in a 40:1 molar ratio. A separatory funnel was employed to separate the bottom layer, which has pH ˜7.5. The bottom layer was micro-filtered and then Na₂SO₄, was added and subsequently separated from the resultant biodiesel product.

EXAMPLE 3 Characterization of the Product

The acid value of the biodiesel product was determined using a titrated oil sample mixed with 70% 2-propanol. The standardized titration procedure with 0.1M KOH was employed.

The biodiesel product was also checked for glycerol by Iodiometric titration. The glycerol in the biodiesel reduces periodate to iodate. The iodate or any remaining periodate is determined by reaction with thiosulfate. The decrease in the quantity of thiosulfate after the reaction is compared with a control, where the same initial quantity of periodate was reacted with thiosulfate. The results indicate the amount of glycerol present in the biodiesel. The total glycerol in the biodiesel of present invention was determined to be less than 0.25 wt % of the biodiesel, with free glycerol being less than 0.02 wt % of the biodiesel.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meanings of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A method for producing biofuel from tobacco biomass, comprising steps of: extracting oil from the tobacco biomass with a polar organic solvent selected from methyl acetate and ethyl acetate, removing the polar organic solvent, transesterifying the extracted oil, and separating the biofuel from the products of the transesterification step.
 2. The method of claim 1, further comprising the step of: comminuting the tobacco biomass before the contacting with polar organic solvent.
 3. The method of claim 1, wherein said organic solvent is methyl acetate.
 4. The method of claim 1, wherein the amount of polar organic solvent is from about 1-4 grams per gram of biomass.
 5. The method of claim 1, wherein the extraction is carried out over a period of from about 16 hour to about 48 hours.
 6. The method of claim 1, wherein the extraction is enhanced by an extraction enhancing technique selected from the group consisting of sonication, heating, agitation, increased pressure, and/or exposure to radiation energy.
 7. The method of claim 1, wherein the polar organic solvent is removed by evaporation.
 8. The method of claim 1, wherein said transesterification step comprises: contacting the oil with an alcohol in the presence of an acidic or basic catalyst.
 9. The method of claim 8, wherein said alcohol is an alcohol containing no more than four carbon atoms.
 10. The method of claim 9, wherein said acidic catalyst is selected from the group consisting of a sulfonic acid, H₂SO₄, HCl, acetyl chloride, and BF₃.
 11. The method of claim 9, wherein said basic catalyst is selected from the group consisting of KOH, NaOH, NaOCH₃, Na₂CH₂CH₃, and guanidines.
 12. The method of claim 1, wherein the transesterification step is carried out after the solvent removal step.
 13. The method of claim 1, wherein the tobacco biomass is obtained from tobacco grown hydroponically.
 14. The method of claim 13, wherein the tobacco biomass is obtained from Navaho mountain tobacco.
 15. The method of claim 14, wherein the polar organic solvent is ethyl acetate.
 16. The method of claim 15, wherein the transesterification step is carried out using methanol and/or ethanol and a sulfuric acid catalyst.
 17. The method of claim 16, further comprising the step of separating the methanol and/or ethanol from the biofuel.
 18. The method of claim 18, further comprising the step of washing the biofuel with water. 